1. Field of the Invention
The present invention relates to a vertical-type CVD apparatus for forming films, such as silicon nitride films, on semiconductor substrates.
2. Description of the Prior Art
FIG. 9 schematically shows in cross section an example of a vertical-type CVD apparatus, which is a type of conventional CVD apparatus. Numeral 11 denotes an inner tube which is made of quartz, 12 an outer tube which is also made of quartz, 13 a manifold flange, 14 a cover, and 15 an 0-ring. These components constitute a reduced pressure CVD chamber 10. A pedestal 17, which is coupled to a rotational mechanism 16 provided outside the CVD chamber 10, is arranged in the inner tube 11, and a quartz boat 18 is mounted on the pedestal 17. A heater 19 for keeping the CVD chamber 10 at a desired temperature is provided outside the chamber 10 (the temperature distribution within the chamber is as shown in FIG. 10, for example). In the uniformly-heated zone of the CVD chamber 10, a plurality of semiconductor substrates 20 are horizontally arranged parallel to each other and spaced a predetermined distance apart from one another. The manifold flange 13 has an exhaust port 21 to which an external exhaust means is coupled to maintain the chamber at a reduced pressure. The manifold flange 13 also has a plurality of raw gas inlet ports, for example, two raw gas inlet ports 221 and 222, through which gas inlet tubes 231 and 232 are inserted into the CVD chamber. At least one of the two gas inlet tubes (e.g., the tube 231) has a portion extending beside the substrates into the uppermost inner region of the reaction chamber. The gas inlet tube 231 has a closed distal end, and is also provided with gas injection holes 34 along its longitudinal direction. The other gas inlet tube 232 has a portion extending up to the pedestal 17 arranged in the inner tube 11. Reference numeral 24 denotes a transfer mechanism.
In the above-mentioned vertical-type CVD apparatus, raw gases (e.g., dichlorosilane gas and ammonia gas) are introduced into the chamber 10 through the gas inlet ports 221 and 222, whereby a CVD film (e.g., silicon nitride film) is deposited on each substrate 20.
The reason for locating the substrates 20 in the uniformly-heated zone is to prevent silicon nitride films, each being deposited on the substrates 20, from having a variation in the film thickness, based on their position in the boat 18.
As previously described, two independent gas inlet tubes 231 and 232 are used in the aforementioned CVD apparatus. The reason these two tubes are inserted into the interior of the inner tube 11 is to avoid unwanted ammonium chloride from being produced as a result of the aforementioned two raw gases being mixed in a low-temperature atmosphere in the vicinity of the cover 14.
In place of the two gas inlet tubes 231 and 232, a single gas inlet tube 30, shown in FIGS. 11A and 11B, can be used. The distal end of the tube 30 is closed, and the proximal end portion thereof is divided into two branches. An injector nozzle portion 33, which is at a higher positional level than a branch portion 31 of the gas inlet tube 30, has a plurality of gas injection holes 34, and raw gases are supplied independently through two gas inlet ports 321 and 322, which are provided at the distal end of each of the branches. It has been proposed that by using such a gas inlet tube, the raw gases with a uniform concentration can be supplied to the aforementioned uniformly-heated zone (Published Unexamined Japanese Utility Model Application 64-37464, for example).
The variation in the film thickness among the substrates can be controlled by use of the batch processing mentioned above. However, there is a problem such that a variation in essential properties (film quality and composition) of the deposited silicon nitride films is not negligible.
In order to investigate the variation in the properties of the silicon nitride film among the substrates, the hydrofluoric acid (HF) etching rate and the refractive index of the silicon nitride film relative to the position of the substrate 20 (the distance from the inlet port of the reaction chamber) were measured. The result of the measurements is shown in FIGS. 12 and 13. As can be clearly understood from these Figures, the composition of each silicon nitride film differs considerably from one another according to substrate positions.
The CVD films formed on substrates should ideally have a uniform thickness and composition. However, in the case of batch processing, as characteristically performed by the conventional vertical-type CVD apparatus, forming CVD films with a uniform thickness is the main consideration; the variation in film composition among the substrates is disregarded.
When the composition of the CVD films is intended to be uniform among the substrates by using the conventional CVD apparatus, the desired results can not be obtained even if the atmospheric condition and the flow rate of raw gases are changed. Therefore, it is necessary to keep the uniformly heated zone at a constant temperature without a temperature gradient. However, such a technique is not used because the throughput of the batch processing, which is a merit of the CVD apparatus, is remarkably reduced.
The problem described above may be somewhat improved if the gas inlet tube 30, whose proximal end portion is divided into two branches as shown in FIGS. 11A and 11B, is employed. However, if the gas inlet tube 30 is used, the thickness of the deposited CVD film is different between the central portion of the substrate and the peripheral portion. That is, the difference of the film thickness is considerably large within the plane of the substrate. This is because each of the gas injection holes faces toward the central axis of its corresponding substrate.
As described above, a conventional vertical-type CVD apparatus has the merits that CVD films can be formed on a plurality of substrates in a single batch and the films thus formed are of uniform in thickness, whereas it has the drawback that the characteristics of the formed CVD films differ.